The distinction between the positive and negative phases of movement is not merely academic. Instead, it is based upon a mechanical difference of the human playing apparatus. A negative movement has no external resistance to overcome, the resistance of friction among the parts moved, being internal.
A positive movement does overcome external resistance. In lifting the finger, its own weight (plus friction at the joints) is the only opposition to the movement, and the movement does no external work. In finger-descent, on the other hand, the aim of the movement is key-depression.
It is necessary in all practice to keep this distinction in mind. If finger-lift be considered the positive element, the movement results in an excessive lift with the extensors straining against the fully extended tendons and ligaments. (An interesting illustration is given under arm-Legato.)
This represents so much wasted work. It does not lift the finger higher, hence cannot effect the following finger-descent favorably. Normally the pupil's attention should be directed toward the positive not the negative phase of the movement. We shall have occasion to note this repeatedly in discussing the various touch-forms.

The fundamental law of passivity may be stated as a subsidiary of the law of least effort. Whatever work is done by an outside agency in moving any part of the body, relieves the muscles normally responsible, and as a result , they relax, and the physiological nature of the movement changes, though its external appearance remains the same.